Was speaking to del who runs the Scottish terrier racing team on Sunday and he showed me one he made. Just the rear half of a bike with the cog welded so you can pull both ways. You put a pulley at the other end and can run them backwards and forwards.
Other option is mechanical. Starter motor, solenoid, spool and a frame to put them on.
Final option is to pm graham4877 and buy one off him. They're the cheapest around and great machines.